Communication links, such as buses, must have proper conditioning to impart to the links such electrical characteristics as are required for communications to proceed over the links. Such conditioning is commonly provided by link terminating apparatus. As the name implies, that apparatus is connected to the end of a link. A bidirectional link is terminated at both ends; a unidirectional link only requires termination at the destination end. The terminating apparatus generally provides proper impedance to prevent reflection of signals from the ends of the link, and also connects a power source and/or sink to the link to power the link and maintain it at the proper operating voltage.
Certain types of systems, particularly input and output (I/O) systems, use a daisy-chained bus as a communication link between a processor and a plurality of devices such as I/O devices. A daisy-chained bus is one which interconnects communicating units in such a manner that physical disconnection of any one of the units effectively interrupts the bus and separates it into two disjointed segments. Connection of units to such bus and disconnection of units from the bus therefore adds segments to, and removes segments from, the bus. Hence, the ends of the bus change with the connection and disconnection of units. Therefore, provision must be made for disconnection of bus terminating apparatus from the old ends of the bus and connection thereof to the new ends of the bus when the system is reconfigured. This is commonly accomplished by including terminating apparatus in two of the units each of which is always connected to one end of the bus upon system reconfiguration. Each of the two units terminates bidirectional bus signal lines and those unidirectional signal lines whose destination end is connected to the unit.
Since termination apparatus is included in a unit, it uses the power source of that unit as the power source to which it couples the bus to maintain it at the proper operating voltage. This creates a problem, however, in that whenever this unit is powered down for any reason, such as for maintenance, the source of power to the bus is lost and the terminating apparatus can no longer perform its prescribed function. Furthermore, the powered-down power source typically offers a low impedance path to ground, and thus tends to ground the bus. As a consequence, the bus becomes inoperative and hence the whole system becomes disabled while the one unit is powered down. It is desirable to maintain communications capabilities between the remaining powered-up units, but the commonly-used termination arrangement does not make that possible.